1. Field of the Invention
This invention relates to a process for producing a toner used in a process for rendering latent images visible and a recording process of toner-jet system.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, etc. are known as electrophotography. In general, using a photoconductive material, copies are obtained by forming an electrostatic image on the photosensitive member by various means, subsequently developing the latent image by the use of a toner to form a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and thereafter fixing the toner image onto the transfer medium by the action of heat, pressure or solvent vapor. As methods for developing the latent image by the use of toners or methods for fixing the toner image, a variety of methods have been proposed, and methods suited for the respective image forming processes are employed.
Toners used for such purpose have commonly been produced by melt-kneading colorants comprising dyes and/or pigments, into thermoplastic resins to effect uniform dispersion, followed by pulverization using a fine grinding mill, and the pulverized product is classified using a classifier to produce toners having the desired particle diameters.
Reasonably good toners can be produced by such a production method, but there is a certain limit, i.e., a limit to the range in which toner materials are selected. For example, colorant-dispersed resin compositions must be brittle enough to be pulverizable with ease by means of an economically feasible production apparatus. However, such colorant-dispersed resin compositions made brittle in order to meet these requirements tend to result in a broad particle size distribution of the particles formed when actually pulverized at a high speed, particularly causing a problem in that fine particles tend to become included in the particles in a relatively large proportion. Moreover, toners obtained from such highly brittle materials tend to be further pulverized or powdered when used for development in copying machines. Also, in this method, it is difficult to uniformly disperse solid fine particles of colorants or the like in the resin, and some toners cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency of toners at the time of image formation, depending on the degree of dispersion. Accordingly, care must be taken when colorants are dispersed. Also, colorants may separate and cause rupture sections of pulverized particles, and may cause fluctuations in developing performance of toners.
Meanwhile, in order to overcome the problems of the toners produced by such pulverization, toners produced by suspension polymerization are proposed as disclosed in Japanese Patent Publication No. 36-10231, No. 43-10799 and No. 51-14895. In the process for producing toners by suspension polymerization, a polymerizable monomer, a colorant and a polymerization initiator, and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a polymerizable monomer composition. Thereafter, this polymerizable monomer composition is dispersed in a continuous phase, e.g., an aqueous medium, containing a dispersion stabilizer, by means of a suitable stirrer and is simultaneously subjected to polymerization to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization, the toner particles are not required to be brittle, and hence soft materials can be used, and also the step of classification can be omitted, bringing about a great cost reduction effect such as energy saving, time reduction and improvement in process yield.
Toners themselves are also required to be made multi-functional because of copying machines and printers being made in recent years to have a high quality image, to enable full-color formation and to enjoy energy saving. For example, accompanying the achievement of high image quality, it is required to make toner particles have very small particle diameter so as to be adaptable to high-resolution digital systems. Accompanying the achievement of full-color formation, it is desired to improve transparency of OHP images; and accompanying the achievement of energy saving, it is desired to make a shape of toner particles that is effective for incorporating low-softening substances in toners and for improving transfer efficiency to transfer materials so as to be adaptable to low-temperature fixing. As a means for satisfying such requirement, toners produced by polymerization are used.
Meanwhile, suspension polymerization, including toners produced by suspension polymerization, react such that the polymerization causes increases in viscosity. Such increases, which are magnified as polymerization progresses make it difficult for radicals and polymerizable monomers to move, and hence polymerizable monomer components tend to remain in a large quantity. Especially in the case of suspension polymerization toners, components having a possibility of restraining the polymerization reaction, such as dyes, pigments (in particular, carbon black), charge control agents and magnetic materials are typically present in the polymerizable monomer system in relatively large amounts in addition to the polymerizable monomers, and hence more unreacted polymerizable monomers tend to remain.
If components acting as solvents for not only polymerizable monomers, but also binder resins are present in the resulting toner particles in a large quantity, the resulting toners have a low fluidity to make image quality poor and also cause a reduction in blocking resistance. Besides the performances correlating directly with toners, such components may also cause, in addition to the phenomenon of adhesion of toner to photosensitive member (drum), a problem exhibited during deterioration of a photosensitive member, namely memory ghost and faint images, especially when organic materials are used in the photosensitive member. Further, there is a problem that the polymerizable monomer components volatilize at the time of fixing to cause a bad smell.
In order to solve such problems, it is proposed, as disclosed in Japanese Patent Application Laid-Open No. 7-92736, to reduce polymerizable monomers present in toner particles to a residue of 500 ppm or less so that image quality can be improved.
With miniaturization and personal use of copying machines, printers and so forth, the restrictions on apparatus increases; the burden to solve the above problems increases and the concern for environment is greater. Thus, the polymerizable monomers should preferably be made present in toner particles in a residue of 200 ppm or less, and more preferably in a residue of 100 ppm or less.
As a method for providing the polymerizable monomers present in toner particles in a residue of 200 ppm or less, a known means for accelerating the consumption of polymerizable monomers may be used when binder resins are produced by suspension polymerization. For example, as methods for removing unreacted polymerizable monomers, there is a method in which toner particles are washed with a highly volatile organic solvent not capable of dissolving toner binder resins but capable of dissolving polymerizable monomers and/or organic solvent components; a method in which they are washed with an acid or alkali; a method in which a foaming agent and a solvent component not capable of dissolving polymers are put into the polymer system and the resulting toner particles are made porous so that the area for the inner polymerizable monomers and/or organic solvent components to evaporate can be larger; and a method in which polymerizable monomers and/or organic solvent components are evaporated under drying conditions. The method in which polymerizable monomers and/or organic solvent components are evaporated under drying conditions is most preferred because in other methods the toner constituent components may dissolve out because of their poor encapsulation in toner particles and it is difficult to select solvents taking into account the undesired retention of the solvents in the toner.
Improvements have been made on drying toner particles after suspensions, formed upon completion of polymerization reaction, have been solid-liquid separated. For example, Japanese Patent Application Laid-Open No. 63-124055 discloses a method of drying toner particles while suspending them by gas to form a fluidized bed. Japanese Patent Application Laid-Open No. 4-311966 and No. 8-179562 also disclose a method of drying toner particles by means of a fluidized bed dryer.
The above method of drying toner particles by means of a fluidized bed dryer can dry toner particles efficiently. Since, however, the unreacted polymerizable monomers stated above commonly have a higher boiling point than water, they can not effectively be removed unless the removal of water has almost been completed. Namely, they can not effectively be removed unless, upon lapse of a constant rate period of drying, the falling-rate drying has sufficiently taken place. However, once the water has been removed, particles having chargeability like the toner particles may adhere to fluidizing chamber wall surfaces, and the particles having adhered to the fluidizing chamber wall surfaces may further come off in masses, causing problems in that the unreacted polymerizable monomers are removed non-uniformly on the whole and the toner particles are formed in a low yield.
Japanese Patent Application Laid-Open No. 6-324517 also discloses a method of instantaneously drying toner particles after a suspension formed upon completion of polymerization reaction has been solid-liquid separated or while dispersing the suspension as it is, in the form of powder particles in a hot air stream so as to be carried in parallel to the hot air stream.
This method of drying toner particles by a hot air stream enables removal of water efficiently and continuously. However, the unreacted polymerizable monomers stated above can not be substantially removed by instantaneous drying.
Japanese Patent Application Laid-Open No. 8-160662 discloses a method of drying toner particles by means of a vacuum dryer.
This drying method, however, not only requires a very long drying time in order to remove water by evaporation and thereafter further remove the unreacted polymerizable monomers but also, when the water is removed, the vacuum in the apparatus may cause toner particles to tighten and agglomerate to form a compressed state. This compressed state is formed significantly when the water content becomes 5% by weight or less, because of an abrupt increase in agglomerative force acting between particles which is caused by an increase in powder temperature. The formation of this compressed state causes toner particles to adhere or melt-adhere to, e.g., apparatus wall surfaces and agitating blades in the apparatus, so that the apparatus is inhibited from stable operation. Also, the agglomeration of toner particles causes powder lumps, so that external additives do not adhere uniformly to toner particle surfaces when externally added in a post-toner preparation step, to cause a problem in the performance required as developers.